1. Field of the Invention
The invention relates generally to the field of optics, in particular to optical polygons which are used to manipulate light beams for reading, writing, and inspecting various surfaces.
2. Prior Art
A tracking system such as shown in FIG. 1 is widely used in industry. It comprises a carousel C1 rotating on an axis R7 with spinning objects at its periphery. The objects are in pockets which rotate on axes R1 to R6. During operation, an object (such as a bottle, not shown) is rotated with pocket P1 while a laser beam L1 from a laser L2 engraves the bottle's surface with identifying marking lines, such as a bar code, a lot number, an expiration (pull) date, etc. Such engraving may be done, e.g., to mark the date the bottle was filled with a beverage.
Typically carousel C1 rotates until a pocket with a bottle is adjacent laser L2. Then the carousel stops. The object is continuously rotating about its axis R1 with a constant angular velocity omega p. Laser beam L1 is aligned through the object's rotating axis R1 and along broken line segment between R1 and R13. The engraving on the bottle is linear, i.e., the length engraved is directly proportional to the duration of the engraving.
When engraving is complete, the carousel rotates through a short arc with angular velocity omega c so as to move the bottle in the next pocket P2 to the engraving position, adjacent laser L2, whereupon the carousel stops and the next bottle is engraved in a similar manner. (Engraved bottles are removed from and bottles to be engraved are placed upon the carousel in a continuous manner at another location thereon [not shown] in a known manner.)
Since the carousel must repeatedly start and stop, the number of objects engraved per hour is relatively low.
A way to overcome this start-stop movement of the carousel is to rotate a unique mirrored polygon on the carousel's axis with an angular velocity of one-half the carousel's angular velocity and shine the laser beam on the polygon so that it is reflected onto the bottles. This arrangement is described in our above-referenced copending application. Its advantages include unlimited physical size and the freedom of optical alignment, i.e., the laser beam can hit the polygon at any angle. One disadvantage is that a gear train is required to rotate the polygon relative to the carousel.
Another previously explored way to overcome the start-stop movement was to sue a regular polygon mounted on the carousel's axis. The polygon had twice as many facets as objects on the carousel, and it rotated with an angular velocity equal to one-half of that of the carousel. The results were also unsatisfactory since tracking on the surfaces of the objects was non linear, as shown in FIG. 2(a) I.e., equally spaced beams H1, H2, H3, and H4 aligned towards center R13 of the polygon engraved non-equally spaced marks M1, M2, M3, and M4 on the tracking circumference of radius R13' (In FIG. 2(a) the carousel is not shown and the spinning objects are outside the circumference of the tracking radius). This was especially undesirable if a series of equally spaced marks had to be engraved on the bottle, e.g., to provide a bar code for a supermarket checkout scanner, or for an inspection station for reused bottles.